This invention relates to thermoluminescent material and associated uses and associated process for making thermoluminescent material.
"Thermoluminescence" is often used to mean any luminescence appearing in a material due to the application of heat. However, as used hereafter "thermoluminescence" refers to the emission of light due to the freeing of trapped electrons by application of thermal energy.
Many solids that contain luminescent centers often contain one or more types of centers that trap electrons. Upon application of suitable wavelengths of light or application of x-rays, such solids produce free electrons. Upon removal of the energizing light, x-rays, or other radiation, the free electrons may be trapped at an energy level higher then their ground state. If the depth of the trap (that is, the amount of energy required to relese the electron from the trap) is large and the temperature is low, the electron will remain trapped for a long time. However, if the trap depth is sufficiently low, the solid may receive sufficient thermal energy to free such trapped electrons even at room temperature. An electron freed from a trap will return to its ground state and emit a photon.
Thermoluminescene may be characterized as a type of phosphorescence. Phosphorescence is generally considered as delayed light emission after the removal of a source of exciting energy used to cause electrons to assume an energy state higher then their ground level. Such "phosphorescence", often called "afterglow", will be used herein to refer to delayed emission having a duration which becomes shorter with increasing temperature.
Although various thermoluminescent materials have been heretofore developed, practical uses of such materials has been limited due to numerous limitations in the materials themselves. For example, some materials require a relatively high energy input (light used to excite electrons) in order to trap a sufficient number of electrons to provide a relatively low afterglow effect. On the other hand, materials which are more efficient at trapping electrons may have trap depths which preclude room temperature level thermal energy from releasing the electrons and causing the light emission. Although the latter materials may be useful for other phenomena, such trap depths are not desireable or suitable for thermoluminescent material. Materials which have a very low electron trap depth do not store electrons sufficiently securely to allow an afterglow effect to last for an appreciable time after removal of the light or other energy used to excite the electrons. That is, the afterglow effect might be noticeable for a relatively short period of time (under one second), but the emission of light due to room temperature level of thermal energy may not be at all noticeable in such materials after a slightly longer period of time (10 seconds).
The above and other limitations were disadvantageous of previously available thermoluminescent material has greatly limited the possible applications for using the phenomena of thermoluminescence. Additionally, the range of possible uses for thermoluminescent material has been limited by the restrictions in form of various thermoluminescent materials.
As used herein, "thermoluminescent material" shall refer to material having an afterglow which is perceptable by an unaided human eye and having a time duration depended upon the temperature.